1. Field of the Invention
The present invention relates to a freeze-stabilized aqueous silicic acid sol for use as a liquid constituent for phosphate-based embedding compositions employed in dental technology.
2. Description of the Related Art
Embedding compositions have been sued in dental technology for a long time. They are employed to produce refractory molds for the precision casting of metal articles and are typically composed of mixtures of quartz meal, cristobalite, tridymite, and a binder. For use in connection with metals having a low melting point, the binder is usually a calcium sulfate hemihydrate (gypsum). For use in connection with metals having a high melting point range, the binder is usually a phosphate binder composed of an active magnesium orthophosphate.
In phosphate-bounded investments like those mentioned in the present invention, setting rate is influenced by the ratio of calcined MgO and monoammonium orthophosphate. Grain size and reactivity of the used MgO also influence the setting rate. Expansion upon setting may be directed by adding colloidal silica compounds, See for example, (Dentists Desk Reference; 1. Materials, Instruments and Equipment. 1981, 1st Edit.; American Dental Ass. 2. Guide to Dental Materials and devices 1976; 8th Edit., American Dental Ass.) 3. C. P. Mabie, Journ. Dent. Res. 52 (1) 96 (1973), and 4. U.S. Pat. No. 2,222,781.
If water and/or a commercially available silica sol are added to an embedding composition and the mixture is later heated, the components of the binder react according to the equations listed below and set up to form a highly refractory substance: EQU MgO+NH.sub.4 H.sub.2 PO.sub.4 +5H.sub.2 O.fwdarw.NH.sub.4 MgPO.sub.4. 6H.sub.2 O, and (1) EQU 2NH.sub.4 MgPO.sub.4. 6H.sub.2 O.fwdarw.Mg.sub.2 P.sub.2 O.sub.7 +7H.sub.2 O.uparw.+2NH.sub.3 .uparw.. (2)
The setting of an embedding composition is known to be accompanied by a change in volume which may be considerable. The volume change is generally a net increase and is generally due to a combination of expansion due to solidification or setting; thermal expansion, especially expansion of the quartz constituents, particularly cristobalite; and thermal contraction, i.e., cooling of the heated embedding composition.
It is also known in the art that these changes in volume, i.e., expansion capability, can be controlled by changing the concentration of the silica sol constituent. Commercially available silicic sols, i.e., silicic acid sols containing up to 45 weight % colloidal silicic acid in aqueous solution, are therefore frequently used in practice as a liquid constituent for phosphate-based embedding compositions.
Commercially available silicic acid sols are disclosed, inter alia, in Federal Republic of Germany Published Application No. 2,110,058, which corresponds to British Patent No. 1,374,537. In the process disclosed therein, silicic acid sols are not used as a liquid constituent for phosphate-based embedding compositions but as a liquefier for calcium sulphate hemihydrate (gypsum) for products especially suitable for the manufacture of flooring plaster and other building products. These silicic acid sols may be stabilized by the addition of amphoteric metal hydroxides, e.g., hydroxides of aluminum, tin, zinc or lead. At temperatures around the freezing point and below, however, these sols irreversibly precipitate. Even after a single freezing and thawing cycle, a silicic acid sol which has not been freeze-stabilized forms a gel that is incompletely soluble in water. There is practically no redispersion of the colloidal SiO.sub.2 upon thawing.
For that reason, attempts have been made to stabilize silicic acid sols with anti-freeze materials, i.e., substances which lower the freezing point. For example, one manufacturer of silica sols furnishes silicic acid sols which contain monoethylene glycol as the anti-freeze material. Although these glycol-containing sols continue to be stable even after repeated freezing and thawing cycles, they are unsuitable as a liquid constituent for phosphate-based embedding substances employed in dental technology because of their unfavorable influence on the expansion behavior of the embedding composition as it sets.
The freeze-stabilization of silicic acid sols, particularly emulsions of wax and coatings (dye mixtures) containing silica sols, is disclosed in U.S. Pat. No. 2,601,291. This patent discloses making silicic acid sols freeze-stable by adding thereto water soluble primary, secondary or tertiary amines. A quantity of from 0.05 to 2.2 mole of amine is added per 100 g SiO.sub.2 in the sol. After six freezing and thawing cycles, turbidity of the sol is mentioned to have appeared, but no precipitation of the sol is alleged to have occurred. The above-mentioned silica sols, however, are unsuitable for use in phosphate-based embedding compositions for dental technology because the expansion behavior of the embedding composition is influenced in a disadvantageous manner.